This invention relates generally to digital message transmission systems and, more particularly, to common carrier systems utilizing multichannel time division multiplexing that efficiently combine packet or message switched data and circuit switched telecommunication messages.
Recently, various multiplexing arrangements have been introduced that combine digital traffic which is switched in different ways to provide an arrangement for sharing the capacity of digital transmission systems. At this level of hierarchy, in digital transmission systems the digital traffic is already processed to provide effective utilization of transmission capacity. Part of the information is generated using concentration techniques and processed by circuit switching (line switching). Digital data is processed by store-and-forward switching which is also called message switching or packet switching. In circuit switching, devices are first connected to provide a "transparent" channel through which fixed quantities of information are transmitted at fixed intervals irrespective of activity. This type of communication is ideally suited for the synchronous traffic utilized by various telecommunication services. In packet switching, devices share channels to send and receive information in the form of message blocks or packets on a demand basis. The data packets are stored in buffers and transmitted asynchronously when space becomes available. An advantage of packet switched device is that they may remain logically connected to a transmission channel but do not use any of its capacity while not communicating. This type of communication conserves transmission at the expense of message-by-message processing to provide "virtual" channels desirable for data services that transmit information at high rates at irregularly occuring intervals.
The combination of these two different switching processes at the lowest level of hierarchy or at the user level produces a number of considerations, particularly in connection with a loop or ring transmission system. One of many typical applications is in the environment of the business community wherein both telecommunication services and data services are required. There are obvious advantages if both can be conveniently offered by a single common carrier communication system. However, conventional analog telecommunication systems require numerous wire pairs that are inefficient since most are idle at any given time, the wiring is awkward to rearrange, and only suitable for the voice type communication or low data communication rates. Unfortunately, conventional digital subscriber carrier systems are designed for voice generated digital traffic assigned to fixed time slots which render them unsuitable for high data rate transmission.
A primary object of this invention is to provide a digital system robust enough to handle present service offerings for voice and data without compromising performance and have sufficient flexibility to provide conveniently for needs of future service offering as well.
Another object of the invention is to provide a packet communication facility at the user level on a common carrier communication system.
A related object is to provide an efficient time division multiplexing arrangement which may be used alone for the telecommunication services or in combination with data services.